Open any Google app or Google site
Tap or click your profile icon (top right)
Select Sign out
If you don’t see Sign out, select Manage your Google Account, then Security, then Sign out of all sessions
To sign out on a specific device/session: open your Google Account, go to Security, scroll to Your devices, select Manage all devices, choose the device, then Sign out
To remove an account from your device:
Android: Settings → Accounts → Google → select account → Remove account
iPhone/iPad: Settings → Mail → Accounts → select account → Delete Account